The present invention relates to an optical waveguide and a method of making the same, and more particularly relates to an optical waveguide consisting of a photopolymerization material whose refractive index may be adjusted according to the intensity of the ultraviolet light which is radiated thereon and a method of making the same.
A certain aspect of this invention relates to an optical waveguide of a multi mode refractive index distribution type, such as is suitable for connection to a graded index type optical fiber and a method of making the same. Another aspect of this invention relates to an optical waveguide which has a parabolic profile of refractive index in both its cross sectional directions. Yet another aspect of this invention relates to an optical waveguide of a large crosssectional area consisting of a plurality of layers of a photopolymerization material and a method of making the same.
Conventionally, in making an optical waveguide, a high polymer material has been used and by selective photopolymerization of the polymer material a waveguide path has been produced therein. However, an optical waveguide made in this conventional way either has a limited thickness or an uncontrolled refractive index profile because of the attenuation of the ultraviolet light which is effective for the photopolymerization of the material. What is desired in this conjunction is a step index type optical waveguide of a large cross- c sectional dimension in which the index of refraction of the core portion thereof is substantially constant across its cross section. Alternatively, what is desired is an optical waveguide which has a certain controlled graded refractive index profile, for instance a parabolic profile, along both its cross sectional directions. Either one of such optical waveguides has good matching characteristics relative to an optical fiber of a particular type, the former having a good matching relative to a step index type optical fiber, the latter having a good matching relative to a graded index type optical fiber. A bad matching can result in increase of transmission losses, transmission delays and so on due to scattering of light.
Specifically, according to a broad concept of the present invention, an optical waveguide of the graded index type may be made by superposing a mask, which has a mask pattern formed in a stripe whose width corresponds to the width of the optical waveguide to be formed, over a film of polymerizable material which is formed for instance by a casting process, and by then irradiating the combination with rays of ultraviolet light. Thereby, the part of the film which is not covered by the pattern of the mask is illuminated by the ultraviolet light and is subjected to polymerization, thus substantially decreasing its refractive index and forming a so-called clad portion on either side of the part of the film which is covered by the pattern of the mask, which forms a so-called core portion, being itself protected from the effect of the ultraviolet light and so being left in its base or low polymer state wherein its refractive index is high.
On the other hand, when a matching with a step index optical fiber is considered, it is desirable to have an optical waveguide having a substantially flat refractive index profile in both sectional directions. It may be possible to achieve such a profile along the widthwise direction simply by radiating ultraviolet light of two different intensities on a certain photopolymerizable material but not in the depthwise direction because of the fact that the ultraviolet light with which the photo-polymerization material is irradiated for polymerization gets attenuated as it penetrates the photo-polymerization material along its depthwise direction. As a result, in an optical waveguide thus obtained without the benefit of the inventive concept of this invention, the difference in the refraction index between the clad portion and the core portion is substantially smaller in the reverse surface than in the front surface. This, which is caused by the fact that, as mentioned above, the intensity of the ultraviolet light tends to be attenuated towards the reverse surface due to the phase separation of the base material and the monomer in the photo-polymerization regions, damages the capacity of the reverse surface in confining light, thereby causing the increase in the transmission loss of the otical waveguide.